This study concerns the nature of the stimulus represented along the decision axis in the yes/no auditory detection task. Two contrasting interpretations, absolute and difference representation of the stimulus, are tested by raising the carrier tone embedded within the background "noise" to the level of the signal, on occasional "catch" trials. Results indicate that difference detection may be the preferred mode of operation when a carrier tone is present and the task is a difficult one. Implications for the TSD model are discussed in terms of the relative efficiency of the two detection mechanisms.